Oxidative drying inks or compositions (hereafter referred as “oxidative drying inks”) refer to inks which dry by oxidation in the presence of oxygen, in particular in the presence of the oxygen of the atmosphere. During the drying process, the oxygen combines with one or more components of the ink vehicle, converting the ink to a semi-solid or a solid state. The process may be accelerated by the use of catalysts such as metallic salts and/or by the application of a thermal treatment. During conventional printing of oxidative drying inks, the drying process proceeds during a few hours to a few days.
Preferably oxidative drying inks comprise catalysts or driers (also referred in the art as siccatives, siccative agents, desiccatives or dessicators) to set up the oxidation process. Examples of driers include inorganic or organic salts of metal(s), metallic soaps of organic acids, metal complexes and metal complex salts. Known driers comprise metals such e.g. cobalt, copper, manganese, cerium, zirconium, barium, strontium, lithium, bismuth, calcium, vanadium, zinc, iron and mixtures thereof. Typical examples of suitable salts include without limitation anions such as e.g. halides, nitrates, sulfates and carboxylates.
In particular, cobalt salts are widely used as driers for inks and coatings due to their high oxidative efficiency and their robustness, i.e. their efficiency remains high independently of the coating compositions.
Catalysts comprising other metals, such as e.g. manganese, cerium, zirconium, bismuth, calcium, zinc and iron, have been used as catalysts for the drying process of oxidative drying inks. However, their oxidative drying efficiency tends to be weaker as compared to cobalt catalysts. Moreover, these catalysts' robustness is more restricted as compared to the conventional cobalt catalysts.
There is some increasing concern about cobalt containing driers for reasons of health and environment issues. Cobalt compounds are under increasing scrutiny and, due to their suspected reprotoxic activity and to new regulations such as REACH, they are increasingly re-classified in more toxic categories. For instance, the environmental toxicity of the widely used drier cobalt octoate is now indicated as very toxic to aquatic organisms; previously it was classified as N, R51/53; the new assessment of this product toxicity requires a classification as R52/R53 for products between 0.25 and 2.5 wt-% of this cobalt compound.
Furthermore cobalt containing driers, when admixed to coatings or inks are known to have a negative tendency to produce discoloration and/or yellowing of said coatings and inks.
In an attempt to provide new environmental more friendly driers, a variety of compounds have been developed.
Manganese containing compounds have been developed as driers for coatings or inks. E. Bouwman and R. van Gorkum disclose complexes of manganese, pentadione and bipyridyl as driers for alkyd paints, in particular for the oxidative crosslinking of ethyl linoleate (J. Coat Technol Res 4(4) (2007, 491-503). WO 2008/003652 A1 and WO 2011/083309 A1 disclose catalysts based on iron-manganese complexes containing polydentate ligands for air-drying alkyd-based resins. EP 1 564 271B1 discloses driers consisting of a combination of iron and manganese salts of fatty acids. WO 2011/098583 A1, WO 2011/098584 A1 and WO 2011/098587 A1 disclose oxidative drying coating compositions comprising polymers containing unsaturated fatty acid residues and manganese salts complexes as drying catalysts.
The main disadvantage of known manganese salts is their tendency to cause coating yellowing (European Coating Journal 03 (2005), 84).
Vanadium containing driers have been developed as driers for coatings or inks. EP 870 811 A2 discloses vanadium complexes as driers for oxidative drying lacquers. EP 2 014 729 A1 discloses intaglio printing inks comprising a vanadium salt as drier.
EP 1 382 648 A1 discloses complexes of metal such as vanadium, manganese, iron, nickel, copper and cerium, suitable as driers that have furthermore anti-skinning properties.
Recent developments in the field of driers for oxidizing alkyds useful as polymeric binders have been reviewed by Soucek and Wu in Progress in Organic Coatings (2012) 73, 435-454. However, none of these driers is as reactive and universal as the cobalt containing driers known in the art. Alternative driers also frequently tend to produce undesired yellowing and bronzing of the dried coating. Moreover, alternative driers often cause storage stability problem related to skin formation inside the ink container and require the addition of increased concentrations of anti-skinning agents.
Therefore, a need remains for oxidative drying inks comprising environmentally friendly driers exhibiting efficient and robust drying while maintaining good non-yellowing characteristics upon use and time.